BUG: unique() should preserve the dtype of the input

stuarteberg · stuarteberg · commit 979843eda965 · 2019-08-12T14:38:40.000-04:00
diff --git a/doc/source/whatsnew/v0.25.1.rst b/doc/source/whatsnew/v0.25.1.rst
@@ -156,7 +156,7 @@ ExtensionArray
 Other
 ^^^^^
 - Bug in :meth:`Series.replace` and :meth:`DataFrame.replace` when replacing timezone-aware timestamps using a dict-like replacer (:issue:`27720`)
--
+- The returned dtype of ::func:`pd.unique` now matches the input dtype. (:issue`#27874`)
 -
 -
 
diff --git a/pandas/core/algorithms.py b/pandas/core/algorithms.py
@@ -404,7 +404,7 @@ def unique(values):
 
     table = htable(len(values))
     uniques = table.unique(values)
-    uniques = _reconstruct_data(uniques, dtype, original)
+    uniques = _reconstruct_data(uniques, original.dtype, original)
     return uniques
 
 
diff --git a/pandas/tests/test_base.py b/pandas/tests/test_base.py
@@ -396,6 +396,7 @@ def test_value_counts_unique_nunique(self):
                 )
             else:
                 tm.assert_numpy_array_equal(result, orig.values)
+                assert result.dtype == orig.dtype
 
             assert o.nunique() == len(np.unique(o.values))
 

Original file line number	Diff line number	Diff line change
`@@ -156,7 +156,7 @@ ExtensionArray`
`156`	`156`	`Other`
`157`	`157`	`^^^^^`
`158`	`158`	- Bug in :meth:`Series.replace` and :meth:`DataFrame.replace` when replacing timezone-aware timestamps using a dict-like replacer (:issue:`27720`)
`159`		`--`
	`159`	+- The returned dtype of ::func:`pd.unique` now matches the input dtype. (:issue`#27874`)
`160`	`160`	`-`
`161`	`161`	`-`
`162`	`162`
Original file line number	Diff line number	Diff line change
`@@ -396,6 +396,7 @@ def test_value_counts_unique_nunique(self):`
`396`	`396`	`)`
`397`	`397`	`else:`
`398`	`398`	`tm.assert_numpy_array_equal(result, orig.values)`
	`399`	`+ assert result.dtype == orig.dtype`
`399`	`400`
`400`	`401`	`assert o.nunique() == len(np.unique(o.values))`
`401`	`402`